A condition known as glare can occur in communications networks whenever two switching units attempt to establish calls using the same interconnecting path at the same time. One administrative method for minimizing the likelihood of glare, for example in seizing two-way trunks between switching offices, is to seize trunks from one office in descending numerical order, starting with the highest numbered trunk, and trunks from the other office in ascending numerical order, starting with the lowest numbered trunk. However, under typical high traffic conditions, most of the trunks are busy and the likelihood of glare in seizing the few remaining idle trunks is unacceptably high.
U.S. Pat. No. 4,644,528 issued to M. Ahmad et al. on Feb. 17, 1987, discloses an arrangement where two remote switching modules (RSMs) have 23 channels or time slots on an interconnecting transmission facility for inter-module calls. The first RSM is defined to be the controller of time slots 1 through 12 on the transmission facility and the second RSM is defined to be the controller of time slots 13 through 23. Each RSM maintains a time slot status map defining the busy/idle status of each time slot on the transmission facility. If the first RSM is processing a call to the second RSM, the first RSM determines whether one of the time slots 1 through 12 is available by reading its time slot status map. If an available time slot is found, the first RSM assigns it to the call. However, if no available time slot is found, the first RSM informs the second RSM of the need to assign a time slot to a call and the second RSM reads its time slot status map to determine whether one of the time slots 13 through 23 is available. If an available time slot is found, the second RSM assigns it to the call. Although the Ahmad arrangement definitively prevents glare, under high traffic conditions and particularly in circumstances where most of the calls are originated from one of the RSMs, a relatively large percentage of calls will require coordination and control communication between RSMs and a sequential status map search by first one and then the other RSM. This results not only in substantial processing and control message overhead, but also in an increased post-dialing delay.
In view of the foregoing, a recognized problem in the art is the inefficiency of known glare-prevention arrangements, particularly under heavy or highly directional traffic conditions. The problem is further complicated in a distributed control switching system where multiple switching entities are coordinating path assignments through an interconnecting switching network, rather than on a fixed transmission facility between only two endpoints.